This invention relates to a fluorescent display tube that displays digit, symbols, patterns or the like by the luminescene of a fluorescent material, and a method of manufacturing the display tube.
In one type of the prior art fluorescent display tube, as shown in FIG. 1, an insulating substrate 1 is coated with a black insulating layer 2 which surrounds anode segments 3. A fluorescent material 4 is coated on the anode segments 3. Grid electrodes 5 are disposed at positions above the anode segments with proper spacing therebetween and filaments 6 are disposed at a proper spacing from the grid electrodes. Thereafter a glass cover 7 with its inner surface deposited with a transparent electroconductive film is sealed to the anode substrate to form a sealed vacuum container. However, with such a display tube, since the displayed digit or the like are viewed through the glass cover, the field of view is narrow.
In contrast, another fluorescent display tube, in which an insulating substrate is made transparent, an anode segment group disposed on the insulating substrate is made of a transparent material, a fluorescent material is coated on the anode segment and the fluorescent display is observed through a surface of the anode substrate opposite to the surface on which the anode segment is formed, has a wider field of view than the former display tube. Hereinafter, the latter construction is called a back luminescence type.
FIG. 2 is an enlarged partial sectional view showing a prior art fluorescent display tube of the back luminescence type in which electroconductive material for example, aluminum is applied by sputtering on a transparent insulating substrate 11 and the sputtered film is photoetched to form lead patterns 14 and transparent segment electrodes 32. Then the lead patterns 14 are covered by an insulating layer 12.
A fluorescent material such as ZnO:Zn is coated on the transparent segment electrodes 32 by electrophoresis or printing technique to form a layer of fluorescent material 33. The resulting structure is housed in a container to be evacuated (not shown) together with cathode and grid electrodes to be described below.
Electrons emitted from a filament cathode electrode 16 are accelerated by a grid electrode 15 and impinge upon the fluorescent material 33 to cause it to luminesce. Displayed digits and the like are observed through the transparent segment electrodes 32 and the transparent insulating substrate 11. In recent years, however, a variety of types of displays have been desired to be made and request for multicolor display has become stronger. To meet such requirement, it is necessary to change the composition of the fluorescent material for satisfying the customer's request for color of respective displayed patterns. This requires repeated coating operations of fluorescent materials for different color patterns, thus not only increasing the number of process steps but also decreasing yield of satisfactory products, which increases the cost of manufacturing.
Moreover, as it is necessary to change the composition of the fluorescent material for different colors, the brightness levels differ for respective displayed color patterns, which makes it difficult to observe displayed patterns.